1. Field of the Invention
This invention relates to the field of urea-sulfuric acid compositions, and particularly to improved methods of producing stable, highly concentrated urea-sulfuric acid compositions which are useful in a variety of applications. The methods allow sufficient control of reaction parameters to consistently maintain a predetermined product composition and crystallization temperature while avoiding incipient product and/or reactant decomposition and potentially explosive autocatalytic decomposition associated with the highly exothermic reaction. The invention also relates to a unique direct air heat exchange process for cooling the reacting liquid phase without significant emissions to the atmosphere.
2. Description of the Prior Art
Urea is widely used as a topical, sub-surface and foliar fertilizer. Sulfuric acid has also been widely used in the agricultural industry and in other industries for numerous purposes. It is known to be highly corrosive both to metals and animal tissue, including human skin. In the agricultural industry, sulfuric acid has been used as a soil adjuvant, a water penetration improving agent, a herbicide for a wide variety of undesired vegetation, and as a selective herbicide on resistant crops such as onions and garlic.
Previous investigators have observed that urea, sulfuric acid and water can be reacted to form solutions containing mono- and diurea sulfates. This reaction is so exothermic, that it is difficult to control reaction temperature in large volume production plants and it is essentially impossible to control reaction temperature during formulation of the higher acid content compositions, e.g., where the ratio of sulfuric acid to urea is about 1 or greater, with available methods. Furthermore, previous investigators did not recognize either the magnitude or importance of incipient product and/or reactant decomposition or the temperatures at which such decomposition occurs for products having different urea/sulfuric acid ratios. Their methods were not adequate to avoid incipient decomposition, particularly in the higher acid compositions, and they did not recognize the effect of such decomposition on process control or product quality.
D. F. du Toit found that urea formed certain compounds with oxalic, acetic, hydrochloric, nitric and sulfuric acids, and that the resulting compounds were stable in contact with their solutions at 20.degree. C. Verslag Akad. Wetenschappen, 22, 573-4 (abstracted in Chemical Abstracts, 8, 2346, 1914).
L. H. Dalman expanded on du Toit's work by developing the phase relationships between the solid phase and saturated solutions at 10.degree. C. (50.degree. F.) and 25.degree. C. (77.degree. F.) but, as in the case of du Toit, did not develop or disclose methods capable of handling the high heat of reaction involved in large scale industrial processing. "Ternary Systems of Urea and Acids. I. Urea, Nitric Acid and Water. II. Urea, Sulfuric Acid and Water. III. Urea, Oxalic Acid and Water"; JACS 56, 549-53 (1934)
In the article "Adding Plant Nutrient Sulfur to Fertlizer," Sulfur Institute Bulletin No. 10 (1964), the Sulfur Institute discussed the addition of nutrient sulfur to fertilizers and mentioned that urea reacts with sulfuric acid to form two complexes of urea sulfate which are useful fertilizers.
Jones, U.S. Pat. No. 4,116,664 discloses what is referred to therein as a tortuous, multistage process of producing combinations of urea and sulfuric acid in which portions of the sulfuric acid are incrementally added to and reacted with the total amount of urea to be reacted in each of several stages until the total amount of sulfuric acid has been reacted with the urea. The resulting product is unstable and requires further processing. Jones preferably adds water later as required to obtain stability and the desired composition. He discloses that the reaction can be carried out at temperatures of 100.degree. to 225.degree. F. and that if the sulfuric acid is added to the total amount of urea at a rate which is too fast, the temperature goes to about 200.degree. to 225.degree. F. and that a gas is emitted that causes changes in product characteristics such as solidification. The patent states that temperatures of 160.degree. to 200.degree. F. are preferred.
Other writers have discussed methods for controlling the heat of reaction in highly exothermic systems such as the urea-sulfuric acid reaction described by du Toit, Dalman and Jones. For instance, William Lohry, in "Techniques of Manufacturing Hot Mix Suspensions," National Fertilizers Solutions Association "Round-Up Papers," pages 34-38 (1968), disclosed that the exothermic heat of reaction of ammonia with concentrated phosphoric acid can be controlled by either internal or external cooling of the reactants in the reaction vessel, and that it is usually desirable to provide a heel of reaction product in the vessel before adding reactants to prevent drastic variations in product pH.
In U.S. Pat. No. 1,884,105, H. C. Moore disclosed a method for producing salts of sulfuric acid, e.g., by reacting concentrated sulfuric acid with liquid anhydrous ammonia, in which control of the highly exothermic reaction is assisted by adding an initial inventory of product to the reaction zone before reactant addition. In Moore's process, the total amount of sulfuric acid to be reacted is mixed with a quantity of ammonium sulfate previously produced and that mixture is then reacted with liquid anhydrous ammonia.
Similarly, in U.S. Pat. No. 3,459,499, G. C. Mullen, Jr. disclosed a process for ammoniating superphosphoric acid in which, according to Mullen, temperatures are effectively controlled, and diminished product quality associated with excessive reaction temperatures is avoided, in part, by adding a large body of ammonium phosphate product solution to the reaction zone followed sequentially by the introduction of the relatively small quantities of ammoniating fluid and phosphoric acid.
Although these investigators disclosed several characteristics of urea-sulfate combinations, methods of making those combinations and, in general, methods of controlling exothermic reactions, they did not recognize either the magnitude or significance of the incipient decomposition temperature in large volumes of reacting urea and sulfuric acid in the presence of reacted urea-sulfate. Nor did they appreciate that incipient decomposition temperature varies with the urea-sulfate composition, i.e., with the ratio of urea, sulfuric acid and water in the reaction phase, or the effect that exceeding the incipient decomposition temperature has on product composition. They also did not devise or appreciate the need for process conditions required to achieve acceptable reaction rates in large volumes of reacting urea and sulfuric acid while preventing either gross or localized overheating to temperatures in excess of the incipient decomposition temperature. Nor did they recognize several characteristics of urea-sulfuric acid reaction products that make them particularly attractive for certain utilities.
It is therefore one object of this invention to provide an improved method for producing concentrated solutions of urea and sulfuric acid.
It is another object to provide a method for maintaining the temperature at all points in a large volume of reacting urea and sulfuric acid at a level below the incipient decomposition temperature.
Another object is to provide a method for producing concentrated solutions of urea and sulfuric acid in which the reacting liquid phase is cooled by direct air heat exchange without polluting the atmosphere.
Yet another object of this invention is the provision of a method for producing concentrated urea-sulfuric acid solutions containing little or no toxic by-products such as ammonium sulfamate and/or sulfamic acid
Another object is the provision of a continuous method for producing urea-sulfuric acid reaction products in relatively large volumes in which reaction parameters, including reactant composition, reaction temperature, product composition and crystallization temperature are continuously maintained at predetermined values.
Another object is the provision of an improved method for the production of concentrated, stable solutions of urea and sulfuric acid having H.sub.2 SO.sub.4 /urea weight ratios of at least about 1.
Another object of this invention is the provision of improved compositions comprising urea and sulfuric acid reaction products essentially or completely free of sulfamic acid and/or ammonium sulfamate.
Yet another object of this invention is the provision of an improved method for fertilizing crops with improved compositions comprising reaction products of urea and sulfuric acid essentially or completely free of sulfamic acid and/or ammonium sulfamate.
Other objects, aspects and advantages of this new invention will be apparent to one skilled in the art in view of the following disclosure, the drawings, and the appended claims.